I think Surtur should be the big villain of Thor 3, and he should initially be a looming threat in the background while Thor deals with Loki's usurpation of the throne of Asgard. Later in the film, Surtur can emerge as the ultimate threat, which forces Thor and Loki to once again put aside their differences and unite against Surtur.

The Enchantress can be in the film as a supporting villain. She can initially be one of the Asgardians who is helping Loki to seize control of Asgard, and she can later help both Thor and Loki to stop Surtur from threatening the Nine Realms.